Method of manufacturing a blind threaded insert

ABSTRACT

A method of manufacturing a blind threaded insert from metal which method comprises the steps of at least partially forming the insert, by a cold-forming process, other than the internal thread; forming the internal thread; and annealing the appropriate part of the insert to promote later deformation during installation.

[0001] The invention relates to a method of manufacturing a blind threaded insert from metal, which insert can be inserted through a workpiece and then radially expanded to secure it to the workpiece. In this context, “blind” means that the insert can be installed by access to one side only of the workpiece. Such blind inserts are well known in manufacturing industry, and are available, for example, under the names Avdel Large Flange Hexsert and Avdel Thin Sheet Nutsert (the words AVDEL, HEXSERT, NUTSERT and THIN SHEET NUTSERT are Registered Trade Marks).

[0002] Blind threaded inserts are usually manufactured either by machining from bar, or using cold forming techniques and bulk annealing processes, the latter method providing the most economic manufacturing route. The strength of such fasteners when installed by the end user in the application is adequate for light or medium duty. Consequently, for heavy duty applications blind threaded inserts are not used.

[0003] The present invention aims to provide a new manufacturing method for producing blind threaded inserts which have a strength compatible with heavy duty applications.

[0004] In the conventional manufacturing route, there is some increase in hardness and tensile strength in the threaded portion resulting from the one cold working operation which is carried out after the bulk annealing process, but of course any benefit in respect of increased strength of the cold working processes prior to annealing is lost.

[0005] The present invention provides, in one of its aspects, a method of manufacturing as set out in claim 1.

[0006] The present invention provides, in another of its aspects, a method of manufacturing as set out in claim 6. According to this method, the bulk annealing process is used prior or subsequent to producing the internal threads and then the thread portion is induction hardened. In this case the material from which the inserts are manufactured is one which has sufficient carbon or other alloying elements to respond to the induction hardening process.

[0007] Further features of the invention are set out in the other appended claims. The invention includes an insert manufacture by a method according to the invention.

[0008] Some specific examples of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

[0009]FIGS. 1a and 1 b show a blind threaded insert before and after installation in a workpiece.

[0010]FIGS. 2a, 2 b, and 2 c show the manufacturing process routes for the manufacture of blind threaded inserts.

[0011]FIGS. 3a and 3 b, 4 a and 4 b show the configuration of installed blind threaded inserts when a tightening torque is applied to the mating bolt.

[0012]FIG. 6 shows the hardness and corresponding tensile strength of the material in the nut portion of the insert and FIGS. 5a and 5 b show the position on the insert at which these hardness values relate.

[0013]FIG. 7 shows a particular type of workpiece in which inserts of the present invention may be used to advantage.

[0014]FIG. 8 shows an insert installed in such a workpiece, and

[0015]FIG. 9 shows a post-installation process being applied to the workpiece.

[0016]FIGS. 10 and 11 show the hardness along the longitudinal sections.

[0017] In FIGS. 1, 3, 4, 5 a, 8, 9,10 and 11 the insert is shown in longitudinal axial section, and in FIG. 5b in cross-section, the cross-section being taken on the arrowed section line on the longitudinal section on FIG. 5a.

[0018]FIGS. 2a, 4 a and 4 b illustrate the prior art.

[0019] The manufacturing methods in Step 1 of FIGS. 2a, 2 b, and 2 c, are performed using a progressive cold-heading machine. The manufacturing method for producing the internal thread as in Step 3 of FIGS. 2a and 2 c, and Step 2 of FIG. 2b is commonly referred to as roll-tapping. These manufacturing methods are of the type commonly used to make blind threaded inserts and are well known and understood by those skilled in the art.

[0020] Thus, referring first to FIG. 1a the blind threaded insert 11 could be produced by any of the processes shown in FIGS. 2a, 2 b, or 2 c. The example shown is cylindrical and is intended to fit in a suitably sized circular hole in the workpiece 19 (FIG. 1b). In other examples not shown, the outer surface 12 of the insert may be polygonal in cross-section, for example hexagonal or square to fit in a correspondingly shaped hole in the application.

[0021] Zone 1 of the insert shown in FIG. 1a is required to be sufficiently ductile to plastically deform by the action of the installation tool (not shown) to form the blind-side bulb 13 in FIG. 1b. In prior art examples of insert manufacturing process, this ductility is achieved by carrying out an annealing process on the whole of the insert, usually as a bulk process applied to the whole batch as shown in Step 2 FIG. 2a. In the prior art insert manufacturing method shown as Process 1 FIG. 2a, the internal thread 14 FIG. 1a is produced at Step 3 FIG. 2a. By producing it this way after the annealing process, there is some local work hardening in the thread region. The hardness of the insert blank after the bulk annealing process is typically 115 Hv (Vickers Hardness Number), the insert material being typically low carbon steel having 0.1% Carbon. This hardening of the thread is localised to the material near the thread surface. A short distance from the thread surface, the hardness rapidly reduces, as shown on the ‘Process 1’ line in FIG. 6. If an attempt were made to use such an insert in heavy duty applications where the mating bolt requires a high level of tightening torque, then because the strength of the material at this section (as shown in FIGS. 5b and 6) is relatively low, the radial force created by the tightening of the bolt 15 in FIG. 4a against the thread in the insert can be sufficient to cause the nut portion to expand radially as shown 16 in FIGS. 4a and 4 b. This causes a reduction in contact area between the thread on the bolt 17 in FIG. 4b and the internal thread in the insert 14 ¹. The reduction in contact area together with the lower thread hardness of the insert results in thread failure at a relatively low tightening torque. For example, an insert with an M8 thread will fail at a tightening torque of about 40 to 45 Nm.

[0022] With an insert manufactured by a process of the present example (Process 2 FIG. 2b), the work hardening of the material which is created during the wire manufacturing process and when the wire is converted into an insert by the cold forming operation Step 1 and the roll tapping operation Step 2 in FIG. 2b is accumulative, resulting in a high level of hardness as shown in FIG. 6 for Process 2. Thus an insert 18-(FIG. 3a) manufactured by this process can be satisfactorily installed in the workpiece 19 because the band annealing Step 3 (FIG. 2b) provides sufficient ductility to allow the blind bulb 13 to form without cracking and the internally threaded portion 20 of the insert has sufficient strength to resist expansion when the bolt 21 (FIG. 3a) is tightened even to very high tightening torques.

[0023] For example an M8 insert manufactured by a process of the present invention can be tightened to torques in excess of 70 Nm without failure. Such an insert is compatible in respect of tightening torques to grade 10.9 and 12.9 bolts.

[0024] The hardness profile created by the band annealing operation can be varied according to the selected process parameters such as the geometry of the band annealing coil, the machine power settings and the process time. Due to the severity of plastic deformation when the blind bulb is formed it is desirable to have a smooth transition from the annealed zone to the hard zone. Such a profile is shown in FIG. 10, where an M8 insert has a hardness transition between insert flange 28 and the deforming cylindrical section 29 over a distance of approximately 1.5 mm, and a hardness transition between the threaded portion 30 and section 29 over a distance of approximately 2.0 mm.

[0025] In a further embodiment of the invention, an insert which has the requisite ductility in Zone 1 (FIG. 1a) and a high strength in Zone 2 for compatibility with grade 10.9 and 12.9 bolts can be produced using manufacturing Process 3 in FIG. 2c. In this case the insert is manufactured from a medium carbon steel having typically 0.3 to 0.35% carbon. The bulk annealing process Step 2 (FIG. 2c) produces a hardness of typically 140 Hv. At this hardness, the insert has sufficient ductility to sustain the blind side bulbing during installation without fracturing. Step 4 of Process 3 (FIG. 2c) involves the induction hardening of Zone 2 FIG. 1a. This process produces a generally even hardness in Zone 2 of about 350 to 400 Hv as shown in the Process 3 line in FIG. 6.

[0026] The induction hardening process is controlled such that the hardening does not extend into Zone 1, otherwise when the blind bulb is formed it would be prone to fracture at the annealed-hardened junction. For this reason, the induction hardened region does not extend fully the length of Zone 2. For an M8 insert, for example, the unhardened portion of Zone 2 would be approximately 0.5 mm as shown in FIG. 11.

[0027] It would be possible to modify the method of the example by interchanging Steps 3 and 4 of FIG. 2c, so that the internal thread is formed after the induction hardening step.

[0028] An advantage of inserts of the present invention manufactured by Process 2 (FIG. 2b) or Process 3 (FIG. 2c) is that they are particularly suitable for use in workpieces known as hydroformed sections. In such workpieces, shown in FIG. 7, the hole into which the insert is to be installed is punched during the hydroforming process. Because this is a blind operation there is not a die as in conventional hole piercing. This results in a dimpling effect in which the material at the edge of the hole is lower than the top surface of the workpiece. Another feature of punching holes in hydroformed sections is that the pierce slug 23 in FIG. 7 purposely is left attached to the parent workpiece.

[0029] The advantage of using an insert of the present invention will become apparent from the following.

[0030] Referring to FIG. 8, which is a longitudinal section through a blind insert installed in a hydroformed section such as shown in FIG. 7, it can be seen that the blind bulb 13 ¹ at the position on the circumference of hole 22 where it meets the retained slug 23 is partially obstructed by the slug, to the extent that the blind bulb 13 is unable to form evenly. However, because the insert is ductile in this region it is able to plastically deform around this obstruction. Furthermore, in some instances, the dimpling effect on the hole is such that the top surface 24 of the installed insert is below the surface 25 of the hydroformed workpiece.

[0031] In such cases, a tool can be used as shown in FIG. 9 consisting of a drawbolt 26 and a flat anvil 27 in which sufficient pulling force is applied to the insert to cause the dimple in the workpiece to reduce until the top of the insert 24 (FIG. 9) is at the same level as the top face of the workpiece 25. Of course, if the top face of the insert 24 was required to be above the level of workpiece 25 then an anvil which had an annular recess in its front face could be used. It is also feasible to combine the action of the drawbolt and anvil with that of the insert installation tool. The operation of using a drawbolt and anvil to flatten the dimple requires a relatively high force, which force has to be transmitted through the thread of the insert, through the blind side bulb, onto the workpiece. An insert which has high strength of thread combined with high ductility in the blind side bulbing region is ideal for such applications. The insert of the present example meets this requirement.

[0032] The invention is not restricted to the details of the foregoing examples. 

1. A method of manufacturing a blind threaded insert from metal which method comprises the steps of: (a) at least partially forming the insert, by a cold-forming process, other than the internal thread; (b) forming the internal thread; and (c) annealing the appropriate part of the insert to promote later deformation during installation.
 2. A process as in claim 1, in which the high ductility in the part of the insert which plastically deforms during installation is obtained by band annealing that part of the insert.
 3. A process as in claim 2 in which the band annealing produces a controlled hardness contour from the part of the insert requiring high ductility to the part of the insert requiring high strength.
 4. A method as claimed in any of the preceding claims, in which the metal is low carbon steel.
 5. A method as claimed in claim 2, which is modified by the interchange in position of the steps of forming the internal thread and of band-annealing the appropriate part of the insert.
 6. A method of manufacturing a blind threaded insert from metal, which method comprises the steps of: (a) at least partially forming the fastener, by a cold-forming process, other than the internal thread; (b) annealing the insert; (c) forming the internal thread; and (d) induction hardening at least part of the threaded part.
 7. A method as claimed in claim 6, which is modified by the interchange of the positions of the steps of forming the internal thread and of annealing the insert.
 8. A method as claimed in claim 6 or claim 7 in which the metal is medium carbon steel.
 9. A method as claimed in any of the preceding claims, which includes a subsequent surface-treatment step.
 10. A method as claimed in claim 9, in which the surface-treatment step comprises electroplating.
 11. A method of manufacturing a blind threaded insert from metal, which method is substantially as hereinbefore described, and illustrated, the accompanying drawings.
 12. A blind threaded insert manufactured by a method according to any of the preceding claims. 